The present invention relates to a system for controlling secondary air supply for an exhaust emission control system of an internal combustion engine.
An exhaust emission control system provided with a catalytic converter of the three-way type is known. In such a system, the air-fuel ratio of a mixture to be supplied to the engine is controlled to stoichiometry by a feedback control system having an O.sub.2 -sensor for detecting the oxygen concentration of the exhaust gases. The exhaust emission control system is effective in controlling the emission at a steady operation of the engine. However, the O.sub.2 - sensor fails to detect the oxygen concentration during cold engine operation and the three-way catalyst does not effect sufficient catalysis of the exhaust gas constituents. Since, generally, a choke valve of the engine is closed, a rich air-fuel mixture is supplied to the engine. Thus, a large amount of unburned gases is discharged during cold engine operation.
On the other hand, during rapid acceleration by a wide open throttle or heavy load operation of the engine, the air-fuel mixture is enriched, which also causes the discharge of exhaust gases containing a large amount of unburned constituents.
U.S. Pat. No. 3,653,212 discloses a secondary air supply system which consists of a reed valve and a valve responsive to the intake pressure with the aid of a diaphragm. However, this system is not to prevent the excessive enrichment during cold engine operation, but to prevent back-firing during engine deceleration.